1. Field of the Invention
The present invention relates generally to a dampening system for an offset lithographic printing press, and more particularly, to a dampener including a solution recirculator for maintaining fountain solution in the dampener reservoir.
2. Background of the Related Art
A variety of dampeners have been developed for the application of dampening solution in offset lithographic printing. To date, three distinct types of dampeners have emerged. The oldest type is the conventional, or ductor type dampener. The slip-roller continuous type dampener followed, and thereafter, the two-roller continuous type dampener was developed. Most dampeners on the market today are variations of one of these three types.
The conventional type dampener has a pan roller, rotating at a speed less than press speed, picking up the dampening solution from a pan style solution reservoir. The dampening solution is transferred from the pan roller to a set of dampening rollers, the dampening rollers rotating at a speed equal to press speed, by a ductor roller that alternately contacts the pan roller and one of the dampening rollers. A printing plate, on which the image to be printed is etched, is attached to a plate cylinder. After the solution is transferred to the dampening rollers, the rollers smooth out the solution to a somewhat consistent thickness and thereafter deposit it onto the non-image areas of the printing plate. The plate cylinder then rotates the plate past the press's inking form rollers which ink the image areas of the plate.
The slip-roller continuous type dampener also has a pan roller that rotates at a speed less than press speed and picks up the dampening solution from a pan style solution reservoir. Like the conventional system, the slip-roller continuous type dampener includes a set of dampening rollers, rotating at a speed equal to press speed, for smoothing out the dampening solution prior to the solution being applied to the printing plate. However, instead of the solution being transferred periodically by a ductor roller, the solution is transferred by a metering roller which is in constant slipping-contact relationship with the pan roller and one of the dampening rollers.
Both the conventional and slip-roller continuous type dampeners require the inclusion of 5% to 15% alcohol in the dampening solution. The alcohol reduces the surface tension of the solution, thereby enabling these type of dampeners to effectively wet the plate.
An example of a two-roller continuous type dampener is disclosed in U.S. Pat. No. 4,455,938 to Loudon, the disclosure of which is incorporated by reference in its entirety. It includes a form roller having an ink receptive compressible surface pressed against the plate which is attached to the plate cylinder. A metering roller presses against the form roller on a side of the form roller opposite the plate cylinder, forming a line of contact therebetween. The rollers rotate at a speed equal to that which the press operates in a direction which results in the formation of an inward nip--defined as a zone near the line of contact between two rotating rollers toward which the roller surfaces approach. Seals are pressed against the ends of the form roller and metering roller. The `trough` created by the rollers and seals form a solution reservoir in which dampening solution is stored. Solution is metered between the rollers and transferred to the plate cylinder by the form roller as required by the printing plate. The two-roller continuous type dampener has proven to be a substantial improvement over the conventional and slip-roller continuous type dampeners in that it is a simpler design, easier to maintain and repair, requires only infrequent adjustments, and does not require alcohol for properly wetting the plate.
In any dampener, dampening solution must be periodically replenished in the solution reservoir as it is consumed during the printing operation. The most basic method of replenishing dampening solution is by inverting a container of solution over the solution reservoir and positioning the container opening at a level which the solution is to be maintained. Disadvantages associated with this method include adverse chemistry changes in the solution reservoir caused by impurities, such as ink and paper dust, that migrate from the plate cylinder. Also, since the solution remains essentially stagnant in the reservoir, pH "hotspots" develop at remote locations in the reservoir. Furthermore, because the container must be positioned above and relatively nearby the reservoir, compromises must be made when choosing the size and location of the dampening solution container. Finally, such an arrangement makes it difficult to maintain and control the alcohol content of the dampening solution in those systems requiring the use of alcohol.
The advent of solution recirculators has greatly improved conventional and slip-roller continuous type dampeners by eliminating many of the problems associated with inverted container type feeders. Solution recirculators typically include a housing in which a large solution storage tank is enclosed. A pump is attached to the tank for pumping solution through a supply conduit to the pan style solution reservoir. A rigid tube extends through the bottom of the pan facilitating removal of excess solution from the pan. The level of the solution in the reservoir is maintained by an adjustable collar that mates with the rigid tube on the interior of the pan. A return conduit provides fluid communication between the portion of the rigid tube extending from the bottom of the pan and the solution recirculator. The recirculator may include a chiller for lowering the temperature of the solution in the storage tank to reduce the evaporation rate of alcohol that may be included in the solution. Cooling the dampening solution has also been found advantageous for systems not requiring alcohol. Examples of solution recirculators are disclosed in U.S. Pat. No. 3,557,817 to Royse and U.S. Pat. No. 4,300,450 to Gasparrini.
One drawback inherent with prior art solution recirculating systems is that they are not adaptable to two-roller continuous type dampeners. This is primarily due to the fact that existing solution recirculators provide solution at relatively high and varying flow rates. In addition, excess solution in the reservoir is caused to return to the recirculator tank by employing gravity. In a dampener utilizing, for example, a pan type reservoir, these characteristics do not cause a problem. However, in a two-roller dampener these characteristics make solution recirculators unsuitable.
It is clear that there is a need in the art for a dampening system that includes a two-roller continuous type dampener that takes advantage of solution recirculators.